Former member of the Maquis resistance group who became cief engineer of the U.S.S. Voyager. Half-Klingon and half-human, Torres had an aggressive personality and had much difficulty controlling her temper, which she attributed to her Klingon heritage. B'Elanna spent her early years with her parents at the Federation colony on planet Kessik IV. B'Elanna's human father left her and moved to Earth when she was five. She and her Klingon mother subsequently went to live on the Klingon Homeworld, but as a result, B'Elanna wasn't close to either of her parents.

Torres attended Starfleet Academy, but dropped out in her second year because of difficulty with Starfleet discipline. She had been a member of the academy's decathlon team. Torres felt that her sometimes-turbulent stint at the academy was evidence that she was not Starfleet material, but at least one of her instructors, Professor Chapman, was so impressed with her original thinking that he recommended she be accepted if she ever sought re-admission to the academy. She later joined the Maquis and served under Chakotay aboard a Maquis ship, and became a member of the Voyager crew when her ship and the Voyage were stranded in the Delta Quadrant in 2371. While in the Maquis, Torres was a close friend of Seska, not suspecting that she was a Cardassian agent.

After joining the crew of the Voyager she had a violent altercation with Lieutenant Carey, her superior officer in engineering. Nevertheless, Captain Janeway chose Torres over Carey to be the cheif engineer of the Voyager. On stardate 48784, Torres was captured by a Vidiian scientist who used a genotron to split her into two individuals, one fully human, the other completely Klingon. Her Klingon self sacrificed her life so that her human half could live, and Voyager's holographic doctor was subsequently successful in restoring her original genetic structure to her surviving half. The experience helped Torres realize the importance of each half of her personality. Torres once sought Chakotay's help in locating her animal guide. She didn't get along with her guide and tried to kill it.

When fellow Voyager crew member Vorik underwent the Vulcan Pon Farr in 2373, he asked Torres to become his mate. Torres declined, but nevertheless experienced Pon Farr herself, the result of a neurochemical imbalance introduced by Vorik. Torres subsequently attempted to mate with Tom Paris.